Closed caption broadcasting is a broadcast service of separately transmitting text data not merged with a television broadcast image to allow a television receiver to selectively display the text as a caption. In NTSC (National Television System Committee) analog TV broadcast, caption data is transmitted by two bytes each time as analog waveforms at line 21 of first and second fields of a vertical blanking interval (VBI), so that a receiver can display the caption data on its screen. Meanwhile, according to ATSC (Advanced Television Standards Committee) digital television broadcasting standard, two bytes of closed caption data can be inserted into extension_and_user_data( ) structure of a picture header as a sort of user data, and control information for reproducing the closed caption can be defined in an Event Information Table (EIT) of Program and System Information Protocol (PSIP) (See “ATSC Digital Television Standard Part 4-MPEG-2 Video System Characteristics (A/53, Part 4:2007), with Amendment No. 1” and “Digital Television (DTV) Closed Captioning, CEA-708-B”). The receiver can accumulate the caption data byte pairs received through the picture header according to the CEA-708-B standard to interpret and display the closed caption according to the definition or specification made by the transmitting party.
The DTV closed caption (DTVCC) compliant with the CEA-708-B standard will now be described briefly with reference to FIGS. 1 through 5.
DTVCC data is transmitted according to a data communications protocol of particular format. FIG. 1 shows the protocol stack of the DTVCC. The protocol stack includes five layers: a transport layer, a packet layer, a service layer, a coding layer, and an interpretation layer.
The transport layer represents a mechanism in which the DTVCC data is inserted into a DTV video sub-system and extracted by the receiver, and is defined in the ATSC A/53 and ISO/IEC 13818 standards. In detail, the DTVCC data is divided in a unit of two bytes and transmitted through picture user data of DTV stream, and a signaling descriptor is transmitted through a Program Mapping Table (PMT) and/or the EIT of the PSIP.
FIG. 2 illustrates a syntax of the closed caption data cc_data( ) transmitted through the picture user data. “process_cc_dataflag” is a flag indicating whether closed caption data cc_data( ) needs to be processed. If the flag is set to ‘1’, the closed caption data should be parsed and its meaning has to be processed, while, if the flag is reset to ‘0’, the closed caption data may be discarded. “cc_count” field indicates the number of caption data constructs following this field and can have a value from 0 through 31. Each of the caption data constructs of the cc_count carries sixteen bits of caption data through a pair of “cc_data—1” and “cc_data—2” fields. When “cc_valid” field is set to 1, it indicates that the two caption data bytes that follow are valid. When reset to 0, the “cc_valid” field indicates that the two caption data bytes are invalid. “cc_type” field denotes a type of two caption data bytes as defined in the CEA-708-B standard. For example, if the “cc_type” field has a value of 10, it indicates that the two caption data bytes are channel packet data, and if the “cc_type” field has a value of 11, it indicates the start of new channel packet data.
Referring back to FIG. 1, prior to be encoded in the transport layer, the DTVCC data is packetized into caption channel packets according to a certain rule. The packet layer is defined by the caption channel packets. FIG. 3 shows the structure of the caption channel packet. An n byte caption channel packet consists of a one byte packet header and n−1 byte packet data. The packet header contains a sequence number and packet size data. The sequence number is three byte data rolling within a range of 0 to 3 in order to determine whether or not a packet has been lost.
The DTVCC caption channel may be divided into a set of logical sub-channels called “services.” Service data are inserted into a caption channel data stream according to a time division multiplexing scheme. The service layer defines a header for closed caption channel service numbers, a service type, a service attribute, and the like. The caption channel stream can carry six standard services and up to 57 additional extended services, and thus allows 63 total services. FIG. 4 shows the structure of a service block with respect to each service. A service block includes a service block header and service block data. The service block header includes a service number and service block size data. If the number of services is 6 or smaller, the service block header has one byte, while if the number of services exceeds 6, the service block header has two bytes.
Referring back to FIG. 1, the coding layer describes how data are coded for the closed caption services. In other words, the coding layer defines the assignment of numeric codes for code space control, caption commands, and caption characters and symbols. FIG. 5 shows a code space organization according to the CEA-708-B standard. The code space consists of two code books each having 256 positions, and are divided into four code groups: CL, GL, CR, and GR. The CL group includes 32 codes from 00h to 1Fh, and C0 code set (miscellaneous control codes) and C2 code set (extended miscellaneous control codes) are mapped to this space. The GL group includes 96 codes from 20h to 7Fh, and a G0 code set (a character set) and G2 code set (extended control code set 1) are mapped to this space. The CR group includes 32 codes from 80h to 9Fh, and C1 code set (caption control codes) and C3 code set (extended control code 2) are mapped to this space. The GR group includes 96 codes from A0h to FFh, and G1 code set (Latin characters) and G3 code set (future characters and icon expansion) are mapped to this space.
Basic codes among the four code groups (CL, GL, CR, and GR) are characters, control codes, and commands of C0, C1, G0 and G1 code sets at an upper portion of the code space shown in FIG. 5. C2, C3, G2, and G3 code sets at a lower portion of the code space shown in FIG. 5 can be accessed by using an ‘EXT1’ code (10h) of the C0 code set. That is, by prefixing the codes by the ‘EXT1’ code, extended C2, C3, G2, and G3 code sets are referenced. In other words, two bytes (i.e., ‘EXT1’+basic code) must be used in order to reference characters in the C2, C3, G2, and G3 code sets.
The interpretation layer defines the DTVCC graphic user interface, namely, how caption data are encoded by using the code sets of the coding layer and how the caption data are interpreted when decoded. The interpretation layer deals with a caption screen, window, pen, text, and display synchronization.
According to closed caption broadcasting implemented by such a protocol, words of speakers, lyrics of songs, movie lines translation, online TV guide, an emergency broadcast, and various other text services can be provided. Recently, as closed caption broadcasting tends to become mandatory limitedly in terms of media access rights of the hearing impaired or universal services, the closed caption broadcasting is expected to expand its utilization.
Meanwhile, the advancement of television technology has reached a level of implementing an apparatus for displaying three-dimensional (3D) images, and a full-scale commercialization of a stereoscopic type 3D television is around the corner at the time of filing the present application. In the stereoscopic 3D display system, two images captured by use of two image sensors spaced apart by about 65 millimeters like the human eyes are transmitted as broadcast signals to a receiver. Then, the receiver allows the two images to be inputted to the left and right eyes of a user to simulate the binocular disparity producing deep perception or stereoscopic.
In order to implement the closed caption in such a stereoscopic 3D television, it is necessary to reveal the closed caption on both the left and right images. If, however, the caption is displayed at the same position of the left and right images in a state that all the objects within the left and right images are displayed with disparity to simulate binocular disparity, the caption display will be incomplete and break down the 3D display to confuse the viewers and result in a headache or nausea.
Even though the ATSC digital television standard (A/53) and its supplemental standard, i.e., the Program and System Information Protocol (PSIP) standard (A/65C) and the DTVCC standard (CEA-708-B) describes transmission of closed caption, the specification of these standards are merely suitable for transmitting closed caption data for common two-dimensional television, not applicable for the 3D television. If 3D caption data are to be transmitted according to these standards, the caption data for each image of the 3D image pair should be transmitted separately, which results in a substantial reduction of available bandwidth by a half causing a problem of inefficient use of resources.
Thus, a method for effectively displaying closed captions compatible with 3D images in a receiver without degrading the efficiency of bandwidth resources is required.